Gasoline, alcohol, and mixed fuel of gasoline and alcohol are used as fuel for an internal combustion engine in order to reduce CO2 emissions and utilize petroleum substitute fuel. When the mixed fuel of which alcohol concentration has varied is supplied to a fuel tank, the alcohol concentration of the fuel in the fuel tank is varied. Since the stoichiometric air-fuel ratio is different between gasoline and alcohol, when the alcohol concentration is varied in the fuel tank, the stoichiometric air-fuel ratio of the fuel is also varied. Thus, it is necessary to vary the fuel injection quantity (actual air-fuel ratio) according to the alcohol concentration of the fuel.
JP-2006-152990A shows an engine control system in which an alcohol concentration sensor detecting alcohol concentration of the fuel is provided in a fuel passage between a fuel tank and a fuel injector.
However, such an alcohol concentration sensor increases costs of the system. In order to restrict an increase in costs, JP-2007-9903A (WO-2006/129198A1), JP-2005-48625A, and JP-2008-14160A show an engine control system in which an alcohol concentration of the fuel combusted in an internal combustion engine is estimated based on any one of an air-fuel ratio feedback control condition (for example, an air-fuel ratio feedback correction value, a deviation in air-fuel ratio, a ratio between a target air-fuel ratio and an actual air-fuel ratio, and the like), a fuel pressure increasing speed during starting period of the engine, a combustion stability (fluctuation in engine speed), an engine torque, and the like.
Besides, a highly electronically-controlled engine control system, as shown in JP-2008-38785A (US-2008/0040018A1), is provided with diagnosis functions, such as fuel system abnormality diagnosis, catalyst deterioration diagnosis, air-fuel ratio control diagnosis, combustion condition diagnosis (detection of misfire) and the like.
In a case that an engine control system provided with an alcohol concentration sensor has diagnosis functions described above, following problems will arise. When a fuel of which alcohol concentration is different from previously filled fuel is supplied to a fuel tank so that the alcohol concentration of the fuel in the fuel tank is varied, after starting the engine, the fuel remaining in a fuel passage between the fuel tank and the fuel injector is initially supplied to the fuel injector. Then, when the remaining fuel is consumed and the fuel in the fuel tank is supplied to the fuel injector through the fuel passage, the alcohol concentration of the fuel injected from the injector is rapidly changed, whereby an air excess ratio (=air-fuel ratio/stoichiometric air-fuel ratio) will be rapidly changed in a lean direction or a rich direction.
Since the alcohol concentration sensor is disposed in the fuel passage between the fuel tank and the fuel injector or in the fuel tank (for example, in a fuel pump), during a period from a time when the alcohol concentration sensor detects varied alcohol concentration to a time when the fuel remaining in the fuel passage between the alcohol concentration sensor and the fuel injector is totally consumed, the alcohol concentration detected by the alcohol concentration sensor is different from the alcohol concentration of the fuel actually injected from the injector.
The fuel system abnormality diagnosis, the catalyst deterioration diagnosis, the air-fuel ratio control diagnosis, and the combustion condition diagnosis is respectively performed based on parameters which vary due to the alcohol concentration of the fuel. Thus, when the alcohol concentration detected by the alcohol concentration sensor is different from the alcohol concentration of the fuel actually injected from the injector, there is a possibility that an erroneous determination may be made in each diagnosis. Such problems arise also in a case that a fuel weight sensor detecting a weight degree of fuel (heavy/light) is provided in a fuel passage or a fuel tank and the varied weight fuel is supplied to the fuel tank.
In a case that an engine control system provided with an alcohol concentration estimating device has diagnosis functions described above, following problems will arise. The fuel system abnormality diagnosis, the catalyst deterioration diagnosis, the air-fuel ratio control diagnosis, and the combustion condition diagnosis is respectively performed based on parameters which vary due to the alcohol concentration of the fuel, such as an air excess ratio (=air-fuel ratio/stoichiometric air-fuel ratio), a fluctuation in engine speed and the like. Thus, when each of the diagnoses is performed, information about alcohol concentration of fuel is necessary. However, since it takes a long time for an engine driving condition to become a stable condition where the alcohol concentration can be estimated, the alcohol concentration can not be estimated for a while after starting of engine. For example, in a case that the alcohol concentration is estimated based on an air-fuel ratio feedback control condition (for example, an air-fuel ratio feedback correction value, a deviation in air-fuel ratio, a ratio between a target air-fuel ratio and an actual air-fuel ratio, and the like), the alcohol concentration can not be estimated until an exhaust gas senor (air-fuel ratio sensor, or oxygen sensor) in an exhaust passage is activated to perform an air-fuel ratio feedback control.
To avoid such an inconvenience, it can be conceivable that a previously estimated alcohol concentration is tentatively used as a current estimated alcohol concentration until the engine driving condition becomes stable. However, until the engine driving condition becomes stable, each diagnosis is performed based on an incorrect alcohol concentration. Thus, an erroneous diagnosis may be performed. Such problems arise also in a system where a weight degree of fuel (heavy/light) is estimated and specified diagnoses are performed based on the weight degree of fuel.